CMOS image sensors according to the related art have a structure in which a photodiode and transistors are horizontally disposed. Although disadvantages of CCD image sensors have been overcome in horizontal CMOS image sensors according to the related art, unsolved problems still remain with horizontal CMOS image sensors.
In horizontal CMOS image sensors according to the related art, the photodiode and the transistor are horizontally adjacent to each other on a substrate. Thus, an area for the photodiode is required in addition to the area for the transistors. This reduces “fill factor”, and limits resolution capabilities.
In horizontal CMOS image sensors according to the related art, it is also very difficult to optimize a process of simultaneously manufacturing the photodiode and the transistor. In particular, a shallow junction is required for low sheet resistance in a rapid transistor process. However, such shallow junction is unsuitable for the photodiode.
In horizontal CMOS image sensors according to the related art, if on-chip functions are added to the CMOS image sensor, the size of a unit pixel must be increased in order to maintain the sensitivity of the image sensor, or the area of the photodiode must be reduced in order to maintain the size of a pixel. However, if the size of the unit pixel is increased, the resolution of the image sensor may be reduced. If the area of the photodiode is reduced, the sensitivity of the image sensor may be reduced.